Test a method which uses a variable depending on environment?

I have the following test method for an Asp.Net controller action method.

[TestMethod]
public void Get()
{
    var controller = new MyController();
    var result = controller.GetAListOfRecords();
    Assert.IsNotNull(result);
    Assert.IsTrue(result.Count() > 0);
}

And the action is

[Authorize(Roles = "Users")]
public IQueryable<MyModel> GetAListOfRecords()
{
    var user = User.Identity.Name;
    var q = from t in ........
            select t;
    return return string.IsNullOrEmpty(user) ? Enumerable.Empty<MyModel>().AsQueryable() : q;
}

However, the var user = User.Identity.Name will not get current user name in the test class (it will actually be assigned an empty string). Is it possible to set user in the test method?